(1) Field of the Invention
The present invention relates to a technique for connecting a plurality of optical devices, in particular, to a method for connecting respective optical devices using optical paths having polarization-preserving characteristics, and an optical apparatus applied with the method.
(2) Related Art
There have been demanded developments of optical communication systems and optical signal processing systems capable of constructing networks of large capacities and ultra-long distance with an explosive increase of IP data communication demand. In a transmission system adopting a wavelength-division multiplexing (WDM) transmission as a basic technique thereof, it is possible to realize the large capacity transmission and easily perform the division-multiplication with the wavelength as a unit, so that the construction of flexible optical networks that performs multiplication-division of different kinds of services at optical levels, such as, optical cross-connecting (OXC), optical add/drop multiplexing (OADM) and the like. Therefore, the development and manufacturing of transmission apparatus and signal processing apparatus using the above system have been remarkably made.
In these apparatuses, there are utilized many optical devices, such as, an optical switch having functions for switching ON/OFF of light, for attenuating the light, for switching to 1×n, or the like, a wavelength filter that separates a signal light for each wavelength, or the like. Specifically, the optical switch (including an attenuator) is used, for example, for adjusting the levels of respective wavelengths at a wavelength division multiplexing section on the sending side, for ALC controlling by an optical amplifier, for wavelength switching in the OXC and OADM, for switching ON/OFF of light and the like. Further, the optical filter is used, for example, for wavelength switching in the OXC and OADM, for separating the respective wavelengths on the receiving side, for cutting off ASE light and the like.
By forming the above mentioned optical devices on a substrate made of SiO2, LiNbO3 and the like, it becomes possible to achieve the high functions, down-sizing, integration, reduction of electricity, and reduction of cost. The plurality of optical devices integrated on the substrate are used individually in parallel with one another, or are cascade connected in a multi-staged structure to be used, so as to achieve the respective functions thereof. Thus, in a case where the respective optical devices are used individually in parallel with one another, an effect owing to the integration becomes large. Further, in a case where the respective optical devices are cascade connected in a multi-staged structure to be used, it becomes possible to achieve the improvement of extinction ratio, if the optical devices are optical switches, while if the optical devices are optical filters, such as, acousto-optic tunable filters (AOTFs), it becomes possible to achieve the narrow transmission band, the improvement of suppression ratio between other channels, and the improvement of extinction ratio when used as notch filters. Moreover, if optical devices having different functions from one another are cascade connected in a multi-staged structure to be used, it becomes possible to achieve a high function and the like.
For example, in a case where a plurality of optical devices each having polarization dependence are connected to be used, generally, an optical path having polarization-preserving characteristic is used for connecting the respective optical devices. This optical path having polarization-preserving characteristic is the one capable of propagating an input light therethrough while holding the polarization plane of the input light in a certain direction. As a specific example of such an optical path, there is known a polarization-preserving fiber having a PANDA type structure or an elliptical cladding type structure. Such a polarization-preserving fiber is provided with an asymmetric stress supplying section in a fiber section, to cause a state wherein the refractive indexes sensed by the polarization light are different from each other in the axis directions crossing each other.
In a case where the plurality of optical devices are connected by such a polarization-preserving optical path, it is an ideal to perform the connection by completely coinciding the polarization axis (Fast axis, Slow axis) directions of the polarization-preserving optical path with the axis direction of polarization light to be input/output to/from the optical devices. However, in the actual connection of the polarization-preserving optical path with the optical devices, it is difficult to completely coincide the axis directions with each other and thus, certain axis deviation cannot be avoided.
If the axis deviation as mentioned above is caused, as shown in the lower part of FIG. 13, the inter-polarization-mode interference is caused in the polarization-preserving optical path, resulted in the periodic wavelength dependence loss in the transmission characteristics of optical devices. The period of this periodic wavelength dependence loss becomes 1/τ, if a difference between the propagation times of Fast axis and Slow axis of the polarization-preserving optical path is τ. Such a periodic wavelength dependence loss due to the inter-polarization-mode interference in the polarization-preserving optical path causes a change in level of transmission light in an optical filter of band rejection type according to the wavelength, to lead characteristic deterioration, if, for example, a rejection type optical filter (notch filter) is structured by a plurality of optical devices. Further, if the polarization light is input to the polarization preserving optical path in the above connection state, there is caused a problem in that the polarization mode dispersion (PMD) occurs in the optical path.